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"This is an update that had been done successfully previously, but failed in this specific instance in an unexpected way," Arthur de Haan, vice president of Microsoft, wrote in a blog post at the time. "This failure resulted in a rapid and substantial temperature spike in the datacenter. This spike was significant enough ... that it caused our safeguards to come in to place for a large number of servers in this part of the datacenter."

The safeguards blocked users' access to their email and files, and according de Haan, there was a "mix of infrastructure software and human intervention that was needed to bring the core infrastructure back online."